It is well known that a continuously variable gear ratio can be established between an input and an output shaft through the use of sun and planetary gears coupled respectively to the shafts with the planetary gears being surrounded by a ring gear the rotational speed of which is controlled to vary the ratio.
Any means of controlling the speed of the ring gear will equally effectively alter the gear ratio. In one arrangement, the outer edge of the ring gear is provided with gear teeth which mesh with another idler gear, together defining a gear pump. A continuous hydraulic loop with a variable restriction connects the input and output of the resulting pump, and thus by varying the restriction, varying resistance can be applied to the ring gear. This system permits continuously varying the ratios quite effectively, but suffers from certain shortcomings in that there is no feedback mechanism incorporated in this system, nor is there any capability of putting the mechanism into overdrive mode as the ring gear and pump are passive and the ring gear cannot be made to exceed the speed at which it is driven by the other gears. In addition, all the work performed to pump the liquid through a restriction is wasted.